Disable api-version automation in validation pipeline

[JoshLove-msft]

Fundamentally .NET doesn't support generating a different api-version than what is specified in tspconfig.yaml, so we should just disable this feature. This is because we have guardrails in place to ensure that the generated code is reproducible based on the tspconfig.yaml. The api-version passed via release planner is not stored anywhere. We don't support having generated code that doesn't match the commit SHA.

https://github.com/Azure/azure-sdk-for-net/blob/main/eng/scripts/Invoke-GenerateAndBuildV2.ps1#L252-L258

We should just log a warning if it is specified.